This invention relates to a raster output scanner (ROS) electrophotographic printer and, more particularly, to a ROS whose output is corrected for writing beam intensity variations by modifications in the process control electronics.
Prior art printers utilizing a raster output scanner (ROS) typically also utilize a rotating polygon having flat reflective surfaces, or facets, in parallel with the axis of rotation of the polygon. A beam, (or beams if a multiple diode is used) is emitted from a light source such as a helium-neon laser or a diode laser. The light is directed through pre-polygon conditioning optics, modulated according to an input electrical signal, and directed onto the facet surfaces of the rotating polygon. The beams are reflected through a post-polygon conditioning lens system and across the full process width of a photosensitive image plane. In these prior art ROS systems, the pre-polygon conditioning optics are incorporated either in an underfilled facet design where the light beam directed against each polygon facet illuminates only a portion of the facet, or in an overfilled facet design, where the light beam completely illuminates each facet and a small portion of adjacent facets. Comparing the two designs, in an overfilled design the facet size required to produce a given spot size at the image plane is greatly reduced allowing many more facets to be accommodated on the same diameter polygon. This, in turn, permits the scan system to operate at a relatively low rotation rate permitting the use of less powerful (and less expensive) polygon motor drivers. This advantage has, heretofore, been more than offset by two factors: low throughput efficiency and non-uniform illumination at the polygon facets. In order to tolerate the low efficiency (typically 10 to 15%), a higher-powered laser diode is required. The non-uniformity problem results from the fact that the scanning beam, being derived from a laser source, has a Gaussian spot shape which is expanded so that more than one facet of a rotating polygon is illuminated. The expanded beam exhibits an inherent beam intensity variation manifested by a decrease in illumination intensity at both ends of the scan (a condition conventionally referred to as "frown"). As the polygon rotates to scan the spot across the image plane, the amount of light reflected to the medium correspondingly varies, resulting in some degradation of the image.
The above described non-uniformity (frown) is not the only cause of intensity non-uniformities in the output scan. Other possible sources of non-uniformity are charge and development variations within a xerographic printing system; non-uniformities in polygon facet reflectivity, laser power degradation, loss of modulator efficiency and the like. Various techniques are known in the art for accomplishing some degree of compensation for these writing beam intensity variations. One technique incorporates the laser into a feed-back loop and then electronically controls the excitation level. Another technique, disclosed in U.S. Pat. No. 4,400,740, describes a system for combining video image signals with beam intensity signals to provide an input to a modulator port which then regulates beam intensity. Still another technique is disclosed in U.S. Pat. No. 4,727,382 to Negishi et al. An intensity control device for a laser in a laser beam printer is described. The intensity control device stores a first representation of a present light intensity of the laser during its nonscanning mode and further stores a second representation of a user selected imagedensity for a hard copy. See Col. 1, line 67-Col. 2, line 29. It is also known in the art to evaluate the laser source printhead at the time of manufacture and to program, or "burn" in, a ROM to create a "smile" correction to the output signal to compensate for the particular frown output intensity variation. Another type of smile correction is disclosed in co-pending application U.S. Ser. No. 07/426,350 now U.S. Pat. No. 4,978,185 assigned to the same assignee as the present invention, The technique disclosed therein, while providing adequate initial compensation, does not correct for the other non-uniformity factors described above which cause changes over time. While the print head could have a new ROM burned in with a correction at a later time, this is an expensive solution and requires removal to a sub-assembly area.
According to the present invention, a plurality of smile corrections are stored in a ROM. A test mode is periodically enabled in which a test pattern print is produced consisting of a plurality of numbered bands of varying density. Each numbered band is associated with one of the correction curves stored in the ROM. An operator, or tech rep, makes a visual identification of the numbered bands on the test pattern print which appears too have an optimum density level. The operator or tech rep then selects that correction signal curve corresponding to the numbered test pattern band he has identified. If the selected band is different from the previous selected band a new correction signal is sent to the laser changing its output beam intensity to the new level. More particularly, the invention relates to a raster output scanner a combination of a laser source for generating modulated high intensity output beams of radiation in response to input video image signals, means to scan said beams across said photoreceptor to expose said photoreceptor forming a latent electrostatic image thereon, means for developing said exposed latent image and transferring said developed image to an output medium, the combination of, a memory means for storing data relating to a plurality of beam intensity correction signals, each stored signal creating a different varying beam intensity exposure of the photoreceptor, and means for periodically providing test pattern video signals to said laser source causing a test pattern output print to be formed, said test pattern test signals containing a plurality of separate areas, each separate area exposed with an intensity profile corresponding to one of said beam intensity correction signals.